The objectives of this project are to study the properties and mechanisms of regulation of muscle contractions in the hand. The results will be used to evaluate and extend theories of stretch reflex function. Mechanical and electromyographic responses to controlled changes in joint angle will be measured in normal human subjects. The components of the total response are due to (i) passive viscoelastic properties at the joint, (ii) dynamic mechanical properties of muscle, and (iii) reflexes from muscle stretch receptors, and each will be assessed individually. The dependence of each of the response components on the magnitude and time course of the test disturbance and on the torque and angle prior to the disturbance will be studied. This will allow a comprehensive evaluation of the regulatory action of stretch reflexes. In particular, the hypothesis that muscle stiffness is regulated will be tested. In addition, the mechanism of reflex action will be assessed by measuring the reflex compensation for an internal disturbance. The gains of the length and force feedback pathways from muscle spindles and tendon organs, respectively, will be calculated from reflex responses obtained before and after the muscle gain is altered by fatiguing exercise. Thus, the relative contributions of muscle spindles and tendon organs to the total response can be quantitated. The results of this project will provide a comprehensive data base for the clinical evaluation of patients with movement disorders (eg stroke, cerebral palsy, Parkinson's disease). In addition, the techniques developed will be directly applicable to studies directed at understanding the mechanisms of disorders of the stretch reflex. The results can be directly applied to the design of feedback control systems in neuroprostheses for the restoration of lost motor function.